Generally the invention relates to the field known as cathodic arc processing. More specifically the invention focuses on the undesirable aspect of low impedance conditions during the cathodic arc process presenting both methods and apparatuses for recovering from such low impedance conditions upon their occurrence.
The cathodic arc process in general is well known in the art. As early as 1974, it was described in U.S. Pat. No. 3,783,231 to Sablev, et al. The process itself, also referred to as "electric arc deposition," and its historical development was explained in 1986 in U.S. Pat. No. 4,620,913 to Bergman. To the extent the descriptions contained in those patents are relevant to understanding and practicing the process and the components involved, each is incorporated herein by reference. To properly understand the problem addressed through the present invention, it is necessary to generally understand the operation of the cathodic arc process.
As relevant to the present invention, the cathodic arc process is a process whereby the surface of an item is coated (deposition), removed (etched), or whereby the surface of an item is conditioned. In basic form, the cathodic arc process involves a system in which is generated a continuously changing arc or series of arcs to ionize a source material. These ions create a cloud of ionized material which in some circumstances may even be a plasma which is then attracted through use of a bias voltage supply to the surface to be acted upon. The cloud is roughly maintained in equilibrium by balancing the rate at which ions are created by the continuously changing arc or arcs with the rate at which the ions are removed by the bias voltage supply. Although the continuously changing arc or arcs (the "cathodic arc" after which the process is named) is desirable, occasionally low impedance conditions occur across the cloud of ionized material and the bias voltage supply. Such low impedance conditions are not only undesirable but they are very difficult to predict because the entire environment is very dynamic. The degree to which this environment is dynamic is perhaps best understood by realizing that the resistance of the cloud varies by as much as a factor of one thousand. The difficulties caused by this degree of variation are especially evident upon initial start-up of the process during which temperature characteristics and electrical characteristics of the entire system are initialized.
In understanding the field of the present invention, it is important to recognize that the cathodic arc process is unique with respect to other deposition and etch processes such as glow discharge, sputtering, or electron beam processes. In most of these other processes, the source material is not separately generated. Rather, in these other processes a single potential is used. In glow discharge or sputtering processes this single potential acts to create the source material and to direct it to the surface of the item to be acted upon. In electron beam systems the source material is created by the electron beam and is then typically allowed to merely dissipate onto the surface of the item to be acted upon. These differences significantly distinguish the cathodic arc process as it relates to the present invention. While in all these types of processes, the cloud is in a high impedance state immediately after extinguishing the discharge across it, in the cathodic arc process the impedance does not remain high. Rather, it rapidly transitions to a very low impedance condition through the continued action of the cathodic arc. In the other systems, source material does not continue to be created, thus the impedance of the cloud remains high, allowing a restart to be quickly effected In a cathodic arc system, the low impedance condition must be removed before a restart can be attempted. Because of this significant difference in effects, the relevant field of the present invention does not include those areas where any electrical discharge tends toward a more stable process. Rather, the field of the present invention relates to only those thin film processes in which any electrical discharge promotes an environment which tends towards instability.
Similarly, inventions such as discussed in U.S. Pat. No. 3,544,913 and No. 3,546,606 to Anderson involve a totally different environment. As mentioned, those inventions relate to the field of electron beam processing in which an arc is "starved" before development by quickly limiting the current which creates the electron beam. Not only is no effort made in the present invention to catch the condition giving rise to the discharge before it develops by quickly reacting to current variations in the relevant power supply; but, no manipulation of the source creation is involved. The low impedance condition is not "starved" in the present invention, rather it is simply cleared away. This difference in approach is critical because although dissipation of the undesirable anomaly can be very quick in an electron beam system, in a cathodic arc system the dissipation is often slower by as much as several orders of magnitude.
Even within the field of cathodic arc processing, it should be understood that inventions directed to creating the continuously changing, desirable cathodic arc deal with an entirely different circumstance. As mentioned earlier, the arc involved in those inventions is not only desirable but must be continuously maintained. Inventions in this area such as U.S. Pat. No. 4,448,799 to Bergman and U.S. Pat. No. 4,448,659 to Morrison are not relevant because they are directed to devices and circuits for starting, maintaining, and restarting this arc. By contrast, the present invention is directed to methods and devices which not only deal with a different condition, but attempt to stop and recover from any discharge which it might cause.
Until the present invention the cathodic arc process has been attended by unpredictable occurrences of a low impedance condition requiring frequent operator attention. While the unstable nature of the process remains, one extremely undesirable situation--that of the need for complete shut-down of the system due to the occurrence of a low impedance condition has been almost entirely eliminated. Prior to the present invention, those skilled in the art of cathodic arc processing found a need to provide the manpower to physically monitor the cathodic arc processing system during many operating situations This was particularly necessary during initial start-up phases prior to initialization of the system. Upon the occurrence of a low impedance condition, the operator would manually shut-down the cathodic arc or substantially reduce its creation This shut-down involved turning off or substantially reducing the amount of power supplied to drive the cathodic arc or arcs and waiting several seconds until the cloud within the chamber had sufficiently dissipated before attempting a restart Occasionally the restart would also be at perhaps reduced source generation parameters as well. Not only did these approaches to the problem result in delay both in initializing the chamber and processing the item, these approaches also resulted in uneven processing and required additional manpower. Of critical importance is the result that by trying to control the occurrence of low impedance condition by manipulating the cathodic arc or arcs, instabilities in the source material such as temperature variations and the like were actually enhanced. Since these instabilities contribute to the occurrence of the low impedance condition in the first place, the solutions used by those skilled in the art prior to the present invention were counterproductive to some degree.
As can be seen from the approaches used prior to the present invention, solutions to the problem were directed away from automatically acting solutions and were even directed away from manipulation of the bias voltage supply. Instead, the teachings were in the direction of manually affecting the cathodic arc processing system through operator involvement and in the direction of focusing on the cloud's creation through the source power supply rather than the bias voltage supply. The pervasive attitude by those skilled in the art towards focusing on the source power supply as a solution to the problem was a logical approach primarily because it is well understood that, for given conditions, the primary reason the process is so dynamic and variable is that the creation of the cloud by the cathodic arc onto the source material is a highly variable process. Since it is well known that these variations are caused by such factors as movement of the cathodic arc across the surface of the source material, temperature variations within the source material, imperfections within the source material, power variations, and variations in the efficiency with which the cathodic arc creates the cloud, to those skilled in the art it seemed only natural to approach the problem by turning off the source power supply. It was thus unexpected that one could look to the bias voltage supply as a solution to the problem. Those skilled in the art failed to recognize that a solution to the problem could be the bias voltage supply. This trend of thought and teaching in the art also resulted in an acceptance of a manual solution to the problem because it was believed that any solution to the problem required a sufficient elapse of time to allow dispersion of the particular local anomaly within both the source material and the cloud which resulted in the low impedance condition. Again, it was unexpected that the solution to the problem could be accomplished automatically in so short a time frame. An important departure from the teachings of those skilled in the art was the determination by the present inventors not to merely wait for dissipation of the anomalous cloud condition, but rather to drive it away with the bias voltage supply. Those skilled in the art simply believed that the amount of time necessary to achieve sufficient homogeneity in the cloud and to disperse the anomaly in the cloud were the minimum amount of times possible. These aforementioned aspects resulted from the fact that attempts by those skilled in the art failed to recognize the problem as one of clearing the cloud once the undesirable condition had occurred. Rather they believed the problem to be one of preventing the undesirable condition from occurring in the first place. While those skilled in the art had long felt the need to avoid low impedance conditions during cathodic arc processing, they simply approached the problem from an entirely different perspective than that of the present invention and thus were unable to realize the deceptively simple solution which the present invention discloses.
A further effect of the present invention is also the fact that the present invention has greatly reduced the time necessary to initialize cathodic arc process systems for efficient processing. Prior to the present invention, initialization or "heat-up" of such systems took a substantial amount of time (for some systems approximately 45 minutes). A practical benefit of the present invention is that this initialization or heat-up time has almost been cut in half. By entirely avoiding manual shut down due to the occurrence of a low impedance condition, not only are the delays to allow clearing of such a condition avoided, but higher voltage, current, and power settings may be used for faster initialization. Those two effects have synergistically combined to result in the substantially reduced initialization time.